Implantable stimulation devices, such as cardiac pacemakers, are often used to remedy improper heart function. These devices generally provide an electrical pulse to a selected area of the heart that is not (in terms of timing or strength) adequately receiving its natural pulse. Under abnormal cardiac conditions, and particularly cardiac rhythm disturbances, pacemaker therapy is applied to remedy several forms of cardiac arrhythmias (rhythm disturbances) including bradycardias, AV conduction block, supraventricular tachycardias, and atrial and ventricular ectopic arrhythmias.
There are essentially two kinds of pacemakers: single-chamber and dual-chamber. Single-chamber pacemakers are capable of sensing and pacing in only one of the atrium or the ventricle. From a practical standpoint, there are essentially two forms of single-chamber pacing: VVI (senses and paces in the ventricle) and AAI (senses and paces in the atrium).
Dual-chamber pacemakers are capable of sensing and pacing in both the atrium and the ventricle. There are many modes of dual-chamber pacing such as VDD (paces in the ventricle only, senses in the atrium and ventricle), DVI (paces in the atrium and ventricle, and senses in the ventricle only), DDI (senses and paces in both the atrium and ventricle), and DDD (senses and paces in both the atrium and ventricle, with an inhibited and triggered response to sensing).
A letter "R" is sometimes added to these pacemaker modes to indicate the pacemaker's ability to provide rate-modulated (also sometimes called rate-responsive or rate-adaptive) pacing in response to input from an independent sensor. For instance, a DDDR pacemaker is capable of adapting to the need to increase a patient's heart rate in response to physiologic stress in the absence of intrinsic response from a patient's sinus node.
A pacemaker uses a lead system to perform its sensing and stimulation functions. A lead system typically comprises at least one lead, one or more conductor coils, and one or more electrodes. The lead is the insulated wire used to connect the pulse generator of a pacemaker to the cardiac tissue. The lead carries the output stimulus from the pulse generator to the heart and, in demand modes, relays intrinsic cardiac signals back to the sensing circuitry of the pacemaker. Typically, a single-chamber pacemaker requires one lead, whereas a dual-chamber pacemaker requires two leads (one for the atrium and another for the ventricle). The conductor coil is the internal core of the pacing lead through which current flows between the pulse generator and the electrodes.
A lead may be unipolar or bipolar. A unipolar lead is a pacing lead having one electrical pole external to the pulse generator, which is usually located in the heart. The unipolar lead has one conductor coil. The electrical pole is typically a stimulating cathode (i.e., negative pole) at the distal tip of the lead. As used herein, a distal end of the lead is the end which is farther away from the pacemaker. A proximal end of the lead is the end which is connects to the pacemaker. The cathode is the electrode through which a stimulating pulse is delivered. The anode electrode (i.e., positive pole) is the case, or housing, of the pacemaker. A stimulating pulse returns to the anode using the body tissue as a return current path. A unipolar lead is relatively small in size and is theoretically more reliable than a bipolar lead. However, a unipolar lead/pacing system is more susceptible to interference by other electrical activity in a patient's body, such as inhibition due to myopotentials, and further may be prone to pectoral stimulation.
On the other hand, a bipolar lead is a pacing lead with two electrical poles that are external to the pulse generator. The bipolar lead has two conductor coils. The stimulating cathode is typically at the distal tip of the pacing lead, while the anode is an annular (i.e., ring) electrode which is few millimeters proximal to the cathode. As such, bipolar leads are less prone to pectoral stimulation. A bipolar lead has better signal-to-noise ratio than that of a unipolar lead, and thus, is less susceptible to interference from myopotential inhibition.
In practice, the cathode (i.e., stimulating) electrode is typically placed in contact with the heart tissue in order to stimulate the cardiac tissue. The anode electrode, however, does not need to be in contact with the heart tissue, since blood tends to conduct electrical currents better than the tissue itself. Nonetheless, it is preferable to have the sensing electrode in contact with the heart tissue to allow the detection of more distinct signals. For more details on bipolar lead structure and electrode placement, reference is made to U.S. Pat. No. 5,522,855, which is commonly assigned and issued to Hoegnelid on Jun. 4, 1996, and is incorporated herein in its entirety by reference. Moreover, for details on quadrapolar (four electrodes) lead structure and electrode placement, reference is made to U.S. Pat. No. 5,304,219, which is commonly assigned and issued to Chernoff et al. on Apr. 19, 1994, and is incorporated herein in its entirety by reference.
While bipolar leads are reknown for their improved sensing characteristics, some physicians still prefer unipolar leads since the additional stiffness of the bipolar leads makes them handle differently. Programmable polarity has the known advantage of permitting physicians the ability to implant the leads of choice and stock only one pacemaker model that can handle both leads. Further, if bipolar leads are initially implanted, the polarity can be modified based on the patient's needs.
There has been a long felt need to simplify the implantation of dual-chamber pacemakers by using only one lead, commonly referred to as a "single-pass" lead. The earliest known single-pass leads was a "multi-polar" device (1974) by Berkovits (U.S. Pat. No. 3,825,015,) in which two electrodes were place in the ventricle and four electrodes were placed in the atrium, however, only the best two of the four atrial electrodes were used ultimately.
"Quadrapolar" leads (1975) were attempted by Woollons et al. (U.S. Pat. No. 3,903,897) in which two electrodes were located in the apex of the ventricle and two "floating" electrodes in the atrium. However, these leads were very stiff, and positioning the atrial electrodes to make contact were difficult.
Both of these systems were extremely stiff, and either had poor contact with atrium or had extremely large, complicated connectors.
One of the simplest single-pass leads was a two-electrode lead (1979) by O'Neill (U.S. Pat. No. 4,154,247) is which electrode was placed in each of the atrium and the ventricle.
Of course, pacing thresholds were also improved upon by forcing the atrial electrode(s) to make direct contact with the cardiac tissue. This may be achieved by either pre-forming the lead in the region of the atrial electrode (as in the '247 patent, supra) or by using various anchoring or active fixation techniques by Grassi (U.S. Pat. No. 4,624,265) and Hess (U.S. Pat. No. 4,664,120).
Later, "tripolar" electrodes were developed with two electrodes in the apex of the ventricle and a single electrode in the atrium (see U.S. Pat. No. 4,585,004, Brownlee). Other attempts at tripolar electrodes included a single electrode in the ventricle, with two electrodes in the atrium (see U.S. Pat. Nos. 4,711,027, Harris; 4,962,767, Brownlee; and also 5,172,694, Flammang).
These tripolar leads of the prior art are chosen since they permit synchonicity between the atrial and ventricular chambers of the heart while providing a less stiff lead, with a smaller proximal connector (which affords a small, less complicated connector on the stimulating device) and without resorting to implanting an additional lead.
However, these prior art leads do not offer full programmability of the electrode polarity. Nor do they provide a simplified lead structure for ease of manufacture and improved reliability. Accordingly, there is a need in the cardiac pacing technology to offer a lead system which offer programmability, is compactly structured, can be easily placed in a patient's heart and, therefore, is inherently more reliable.